Integrated circuit chips have become increasingly complex and precise through the years. Thousands, and even tens of thousands, of transistors are now disposed on a chip not more than a quarter of an inch (1/4") in the longitudinal and lateral directions. Furthermore, these chips now operate with great precision in nanoseconds of time to perform tasks considered unattainable just a few years ago.
After they have been fabricated, the integrated circuits chips have to be tested. The testing of these chips has become increasingly complex as the integrated circuit chips have become increasingly complex. For example, the testing has to be provided at the same frequencies at which the integrated circuit chip will be operating. For example, the testing has to be often in the range of hundreds of megahertz and even gigahertz. Furthermore, the timing in the operation of the testing circuitry has to be performed with a precision in the order of nanoseconds in order to match the timing precision in the integrated circuit chips.
In testing integrated circuit chips, the times for the production of different signals in the chips have to be determined. Sometimes these signals change from a positive polarity to a negative polarity or from a high magnitude to a low magnitude. At other times the signals change from a negative polarity to a positive polarity or from a low magnitude to a high magnitude. When the signals change polarities or magnitudes, they reach a threshold level where they trigger the operation of stages responsive to these signals. Determinations have to be precisely made of the times when the signals reach this crossover level in either a positive-going direction or a negative-going direction.
Systems have been provided in the prior art for determining when variable analog input signals reach a threshold level in either a positive-going direction or a negative-going direction. The systems of the prior art have had certain inherent problems. One problem is that they respond to positive-going signals differently than negative-going signals. This causes the time measurements of crossover relative to the threshold voltage to be different for positive-going signals than for negative-going signals. Furthermore, the measurements have been relatively slow and have involved a considerable consumption of power. These problems have been recognized for some time. Attempts have been made to provide systems which resolve these problems but the attempts have not been successful.